Simple steps to accurate alignment

In brief:

• Spacer shaft alignment has two unique aspects. The amount of alignment correction at the motor can seem to be very large. And you are correcting two angles, one for each coupling.
• To achieve good, repeatable results, pre-alignment steps must be done. Inspect the coupling for wear prior to alignment.
• Minimize the soft foot under the motor before performing alignment. The difference in a 15-min alignment and an eight-hour alignment is often correcting the soft foot. Utilizing the soft foot function in laser alignment systems works well.

Figure 1. In this cooling tower spacer shaft configuration, the distance from the motor to the gearbox is more than 12 ft.

Many maintenance personnel have experience with aligning standard coupling configurations, such as a motor to a pump, but some are less confident when they’re asked to align cooling tower fans with spacer shafts.

What’s the big deal? It’s almost always too hot, too cold, or too wet. There’s usually lots of steps and ladder climbing involved, most of it carrying big wrenches, hammers, and pry bars. Most cooling towers seem to be built with the assumption that no one will ever need to get to the fan, gearbox, or shaft again, so there may not be a place to stand inside of the cell. And it’s a long way down to the basin.

Cooling towers fans with spacer shafts can be aligned just as accurately and easily as any other coupled machine, if you follow a few simple steps. The same principles apply to most other machines using spacer or line shafts.

Several configurations of cooling tower fan drives exist. A spacer shaft doesn’t need to be treated like a separate machine because it isn’t. It has no bearings or support mechanisms of its own. It must go where the driver and driven shafts take it. If you do happen to have a long spacer shaft with a center support bearing, the center support bearing must be treated like an additional machine component, and aligned as a machine train.

Spacer shaft alignment has two unique aspects. First, the amount of alignment correction at the motor can seem to be very large. While the distances across the coupling faces may be only a few inches, the faces of the couplings, and the shafts, form right angles. So the angles extend for several feet back to the motor or movable machine. When the distance between the driver and driven machines is several feet, the correction amounts at the motor feet can be ¼ in. or more, but the offset and angularity at the couplings may be within tolerance (Figure 1).

Second, you are correcting two angles, one for each coupling. There is really no offset, or parallel correction. If you get all four coupling faces aligned, the spacer shaft will go along (Figure 2).

Pre-alignment steps

As with any type of shaft alignment, to achieve good, repeatable results, pre-alignment steps must be done.

Figure 2. When the angular misalignment is reduced to near zero degrees across both coupling faces in the vertical and horizontal directions, the driving and driven shafts will be aligned.

Inspect the coupling for wear prior to alignment. There is no logic to performing a precision alignment, only to have to replace the worn or damaged components, and then align it again.

Make sure the surfaces onto which the motor and gearbox mount are free of dirt, rust, paint, or other debris that could get under the feet of these components. This will greatly simplify the alignment process, and reduce errors due to debris. Taking two minutes to clean the junk from underneath the motor feet is worth it.

Minimize the soft foot under the motor before performing alignment. The difference in a 15-min alignment and an eight-hour alignment is often correcting the soft foot. Utilizing the soft foot function in laser alignment systems works well. It can also be done manually with feeler gauges or shims in a two-step process.

• With all hold-down bolts loose in the motor, use a shim or feeler gauge to measure each foot in at least three places, to minimize angular and parallel soft foot. Correct to less than 0.005 in.
• With all hold-down bolts tights, loosen one foot, and recheck for soft foot in the same manner, this time correcting angular and parallel soft foot to less than 0.002 in.
• Or it can be done using dial indicators with magnetic bases, if the dial indicator can be located to sufficiently measure any soft foot movement.